A differential refractometer includes a sample cell through which a solution containing a sample is to pass, and a reference cell for a reference solution. These cells are arranged with a light transmissive partition in between. Measurement light from a light source is to sequentially pass through the reference cell and the sample cell. Measurement light is made to diagonally enter the partition between the cells, and the position of the optical axis of light which has passed through both cells is detected by a detector (see Patent Document 1).
The position of the optical axis of light passed through both cells varies depending on the refractive index difference between the reference cell and the sample cell. Since the refractive index of the reference cell is constant, the refractive index difference between the reference cell and the sample cell is changed according to the concentration of the sample solution. Accordingly, by measuring the amount of displacement of the position of the optical axis of light passed through the reference cell and the sample cell, the amount of change in the refractive index of the sample solution may be determined, and the concentration of a component in the sample solution may be determined from the amount of change in the refractive index of the sample solution.